[Bug other/49144] New: [4.6/4.7 regression] miscompiled reverse_iterator?

[Bug other/49144] New: [4.6/4.7 regression] miscompiled reverse_iterator?

[doko at ubuntu dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49144

           Summary: [4.6/4.7 regression] miscompiled reverse_iterator?
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: doko@ubuntu.com


[forwarded from http://bugs.debian.org/PR627795]

seen with 4.6 r174102 and trunk r173903

The attached test program is supposed to print -42, and does so when compiled
with g++-4.4 or g++-4.5. However, it outputs random junk when compiled with
g++-4.6:

$ g++ -Wall -O2 test.cxx
$ ./a.out
31
$ ./a.out
207
$ ./a.out
79


#include <iostream>

int f()
{
    unsigned char data[3] = { 42, 42, 42 };
    unsigned char *begin = data;
    unsigned char *end = data + 3;
    std::reverse_iterator<unsigned char *> reversed(end);
    for (; begin < end; begin++, reversed++)
        if (*reversed != 42)
            return *reversed;
    return -42;
}

int main(int argc, char ** argv)
{
    std::cout << f() << "\n";
}

[Bug tree-optimization/49144] [4.6/4.7 regression] miscompiled reverse_iterator?

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49144

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.05.24 17:30:16
                 CC|                            |rguenth at gcc dot gnu.org
          Component|other                       |tree-optimization
     Ever Confirmed|0                           |1

--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-05-24 17:30:16 UTC ---
Seems indeed a serious issue, note happen only at -O1 and above. Let's ask
Richard to have a look.

[Bug tree-optimization/49144] [4.6/4.7 regression] miscompiled reverse_iterator?

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49144

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-05-24 21:58:09 UTC ---
Seems tree dse2 removes data[0] = 42; store.

[Bug tree-optimization/49144] [4.6/4.7 regression] miscompiled reverse_iterator?

[hjl.tools at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49144

H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |davidxl at gcc dot gnu.org
   Target Milestone|---                         |4.6.1

--- Comment #3 from H.J. Lu <hjl.tools at gmail dot com> 2011-05-25 02:32:27 UTC ---
It is caused by revision 162587:

http://gcc.gnu.org/ml/gcc-cvs/2010-07/msg00941.html

[Bug tree-optimization/49144] [4.6/4.7 regression] miscompiled reverse_iterator?

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49144

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Depends on|                            |48702

--- Comment #4 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-05-25 08:30:28 UTC ---
Sounds similar to PR48702.

[Bug tree-optimization/49144] [4.6/4.7 regression] miscompiled reverse_iterator?

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49144

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Depends on|48702                       |
         Resolution|                            |DUPLICATE

--- Comment #5 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-05-25 10:33:24 UTC ---
I have verified that the patches for PR48702 also fix this bug.

*** This bug has been marked as a duplicate of bug 48702 ***